Abstract

Clinopyroxene is a mineral commonly associated with kimberlite. However, its use as a kimberlite indicator mineral in overburden studies is limited because of its association with many other bedrock types. Historically, the Cr2O3 concentration has been used to identify clinopyroxenes associated with kimberlite intrusions. However, since the Cr2O3 concentrations of clinopyroxene isolated from Ontario kimberlite varies from 0.03 wt% to 3.0 wt% and compositionally overlaps with a variety of other rock types, this approach is ineffective and can be misleading. In an attempt to improve on this technique the role of additional components, such as Al and Na, on the paragenesis of clinopyroxenes is investigated. The entry of Al, Na and Cr into the clinopyroxene structure is controlled by a number of molecules that are discussed in detail. At high pressures (mantle depths) these elements preferentially enter the clinopyroxene structure as jadeite or kosmochlor molecules. To demonstrate this, clinopyroxene data from many sources are presented on a ternary diagram representing atomic proportions of Al, Na and Cr. Clinopyroxenes isolated from kimberlites are unique in that the trend of data extends from the mid portion of the bottom axis (low Cr, high Al and Na) to the mid portion of the right axis (low Al, intermediate Cr and high Na). This trend is consistent with a jadeite–kosmochlor control on the entry of these elements into the mineral structure. A template defining those clinopyroxenes associated with a deep mantle paragenesis and, therefore, an association with kimberlite is presented.

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